Mitchell



Dec. 26, 1950 D. MITCHELL TRANSMISSION SYSTEM 3 Sheets-Sheet 1 originalFiled May 13, 1941 Q ./NVENTOR o M/ TCHEL l.

ATTORNEY Dec. 26, 1950 D. MITCHELL TRANSMISSION SYSTEM Original FiledMay 15, 1941 3 Sheets-Sheet 2 Il Ill NN .S lll I im w ,Nut a :m

Dec. 26, 1950 D. MITCHELL TRANSMISSION SYSTEM Original Filed May l5,1941 5 Sheets-Sheet 5 /NVE/VTOR D. M/ TCHE L L NN QN bm ...53am ..8kauwgum.

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AT TORNEV Ressued Dec. 26, 1950 UNITED STATES PATENT OFFICE BellTelephone Laborat cries, Incorporated, New

York, N. Y., a corporation of New York Original No. 2.301,223, datedNovember 10, 1942,

Serial No. 393,196, May

13. 1941. Application for reissue February 9, 1950, Serial No. 143,311

9 Claims.

Matter enclosed in heavy brackets appear reissue specification; matterprinted in italics indie This invention relates to a transmission systemand more particularly to a speech transmission system of the typeutilizing a plurality of oneway speech transmission channels.

An object of the invention is the achievement of increased efliciency inthe operation of a speech transmission system by reduction of the idletime of the various one-way speech transmission channels.

Another'object of the invention is the transmission of messages in sucha manner that intelligible reception of the messages by unauthorizedpersons will be rendered diflicult.

A more specific object of the invention is reduction of the frequencyband required for transmission of the signals necessary for control ofswitching operations incidental to the speech transmission.

In accordance with a specific embodiment of the invention, a pluralityof one-way speech channels are associated with a larger number ofsubscribers telephone circuits; for example, six speech channels (threewest-east channels and three east-west channels) may be associated withten different subscribers telephone circuits (five at the west terminaland five at the east terminal). Switching means are provided whereby anidle speech channel is selected for transmission of each differentspeech portion of vocal interval duration occurring in the varioussubscribers telephone circuits. The switching means provided iseffective, when a speech fragment of vocal interval duration is appliedto a speech channel, to preselect the next idle speech channel, that is,the channel to which the speech fragment of vocal interval durationoccurring next will be applied.

It will be obvious that the arrangement is effective as a speech channelsaver from the statement above which points out that the number ofsubscribers telephone circuits served by the speech channels is greaterthan the number of said channels. This makes it possible to takeadvantage of the now well established fact that during a so-calledconversation period on any four-wire circuit, either one-way channel isactually in use only about one-third of the time. Privacy oftransmission is attained by the arrangement contemplated in View of thefact that snatches of several different conversations appear in a randommanner on any one of the various speech channels.

A feature of the invention is switching means effective, when a speechfragment of vocal interval duration isv applied to a speech channel,

s in the original patent but forms no part of this ates the additionsmade by reissue 2 to preselect both the near, or transmitting, end andthe distant, or receiving, end of the next idle speech channel, that is,the channel to which the speech fragment of vocal interval durationoccurring next will be applied. l

Preselection of the distant end of the next channel at the same time thenear end of the channel is preselected is a particularly valuablefeature of the invention as it makes possible a substantial reduction inthe frequency band required for transmission of the signals necessaryfor control of the switching apparatus. Obviously, if a channel savingarrangement is to function with full eiiiciency the frequency bandrequired for the control signals must be kept within relatively narrowlimits. v v Y In general, the term vocal interval as used herein meansthe duration of one or a succession of syllables no one of which isseparated from the preceding syllable by an interval as great as thehang-over time of the system, weak beginnings and endings, i. e.,portions not capable of causing relay operation, being considered partsof 'the silent interval. Hang-over time is a term commonly used inconnection with systems including voice operated devices and it meansthe period that a voice-operated device remains, due to its owncharacteristics, in operated position after the operating impulse hasbeen removed. Hangover time insures that weak endings of syllables, notcapable by themselves of holding the relays in operated position, willnot be lost by clipping and may be increased or decreased in duration asthe type of system demands.

Full understanding of the operation of the arrangement contemplated bythe invention as well as appreciation of the various advantageousfeatures thereof may be gained from consideration of the followingdetailed description in connection with the accompanying drawings inwhich:

Fig. 1 shows diagrammatically a two-way telephone system embodyingfeatures of the present invention;

Fig. 2 shows in ing arrangement associated with detail the transmittingswitch'- one of the subl scribers telephone circuits of Fig. l; and

Fig. 3 shows in detail the receiving switching arrangement associatedwith one of the subscribers telephone circuits of Fig. 1.

Referring now to the drawings there are represented in Fig. 1 twoterminals, west and east, of a two-way telephone system which may be,for example, a two-Way radio or line wire telephone system. Fivesubscribers sets Il, l2, I3, I4 and I5 are shown at the west terminal,each and 25, one West-east control signal channel 2l, three eastwvestspeech transmission channels 4|, 42 and 43 and one east-west controlsignal channel 44.

The West terminal switching apparatus is represented by box 45 and Ythe`east `terminal switching apparatus by box 46; details of theseswitching arrangements will be brought out by subsequent description ofFigs. y2 and 3.

The telephone circuit-of -each of the west'subscribers sets hasassociated therewith an oscillator; each of these oscillators produces adis- .tinctive tone frequency. For example, "the tel'e'- phone circuitof subscribers set II hasassociate'd Ytherewith yoscillator 5I which`produces a tone of `frequency' f1,the telephone circuit of subscribersset I2 has associatedztherewith oscillator 52 'which 'produces atene offrequency f2 and soon. ASimilarly, the telephone circuit of each of thefeast :subscribers sets has associated therewith .an os- 'cillator which.produces a 'tone of the saine ffnequency .as vthat "produced by VtheYoscillator asso'ciated'with the corresponding west subscribers set. Forexample, the telephone circuit of subsoribers .set I6 has associatedtherewith 'oscillator :53 which produces vatone fof frequency tf1, thetelephone circuit of subscrib'ers set Il has associated therewithloscillator '54 "which produces .a ltone of frequency f2 and so on.Oscillator 55 ywhich produces a vtone of frequency "fs is Aprovided `atthe westzterminal and oscillator "Slfwhich :likewise produces a 'tone ofVfrequency fa 4is prol-videdat the east terminal; the :purpose :of thesetones will `be described subsequently.

It Iwill be observed Ythat'in accordance with the arrangementcontemplated by the present inven- "tion yeach yof 'the ycontrol signalchannels .or wpaths 27 and 44 need transmit a frequency iband only ofsuicient breadth `to include six different distinctive tones whereas inprior vsystems it `was necessary to `provide for transmission of a-substantially greater frequency band. This reducltion in 'the controlfrequency 'band results from Vthe :nevel varrahgen'ient whereby not onlytis the `sendir-.g end of the next speech transmission channelpreselected but vthat the receiving rend -of the narse speech ychannelis also preselected.

The operation of the terminal switching 'ar ranmments 45 ard 4S will `bedescribed in detail i=1 connection with FigsZ and 3*; their operationwill be described only briefly at this point. Let us assume that aconversation is taking place -b^tween subscriber A (set it) andsubscriberAl (set iii) and that a speech fragment of nvocal intervalduration originating at set H has `just been anplied'to speech-channel24. Switching arrangement 55 operates at this time Vto do severalthings` among them being the application'to control `signal path 2'! ofthe tenes produced by oscillators 5I and 55. Tone produced by oscillator5i actuates proper control apparatus of east terminal arrangement inorder to cause thespeech fragment Yof vocal interval which itaccompanied to be :applied -to the .receiving Aside `.of the telephonecircuit of subscribers set i5; the .tone produced by oscillator v55performs certain .regulatory functions k.which will .be described.subsequently in reference to Figs. 2 and 3. Switching arrangement 45also acts at this time to preselect the transmitting .end of the idleWest-east speech channel (let us say channel 25) to which the nextspeech fragment of vocal interval duration originated by subscribers setIl or any of the other westsets will be applied. At the same timecertain apparatus of switching arrangement 45 is .conditioned topreselect the receiving end of that same-channel whose sending end waspreselected by 'apparatus of the west switching arrangement.

The next speech fragment of vocal interval -duration originated by anyof the West [subscribers] subscribers sets will be applied to speechchannel 25, this channel having been preselected as described above. Wewill assume that this speech fragmentcomes from subscribers set l2,subscriber B .and subscriber B1 being conversing. At the time the speechfragment is applied to speech channel 25 tone f2 produced by'oscillator'52 -is applied to lcontrol Vsignal .path y2'l. This tonecauses operation V*of certain apparatus .of :switching `arrangement -46effective to Vapply the speech fragment-ofvocal interval durationIcoming 1in over channel 25 to be applied to Vthe receiving side of thetelephone vcircuit of subscribersfset il. Itshould be observed here thatin view `of the fact that both Yends of Aspeech channel 25 werepreselected it is necessary to transmitonlyone-control signal tone, i.e., a tone to identify the west subscriber, and that nochan- `nelidentifying tone need be transmitted.

Speech fragments of vocal interval duration as they -origi-nate inanyofthe telephone circuits of the west terminal are, therefore, placed on.the .particular west-. eastchannel whichhappened to .be lastpreselected.; .no .two consecutive l.speech fragments of vocal intervalduration can be placed on the same .speech lchannel .in View of thepreselection feature.

The .arrangement functions in a similar manner for .east-westtransmission, thespeech .fragments of vocal interval duration .togetherwith .the respective control tones being applied to speech-channels 4|,42 and 43 in accordance with presLlection of .the sending .and receivingends `thereofby switching arrangements 45 Aand 45, respectively.

t will be clear from the above that it will `be very diiicult forunauthorized persons who may listen in on the speech channels -tointerpret the messages being transmitted thereover as there will occuron .any one of lthe channels snatches of several different conversationstbes; conversae tion-parts appearing in a purely random manner.

It will also be apparent that the arrangement is effective as a speechchannel saver. Thus in the specific syst'm disclosed, only six one-Wayspeech transmission channels and two control signal channels areutilized for interconnection .of ten subscribers. With a larger numberof subscribers it is possible to closely app-roach the ideal conditionof having only one-third as many speech channlls in one direction asthere are subscribers and yet provide adequate facilities forKnninterinpted transmission; fthe feasibility of such an arrangement isindicated by the previously Arecognized fact that during the time tivo.subscribers are connected over a four-wire circuit .either side .of thecircuit is actually in use .only about one-third of the time. Thesubstantial reduction in the frequency band necessary lfor the controlsignals which results from the novel .arrangement of the presentinvention .75 vwhereby both .the sending andthe .receiving end lcepis ofthe speech channels are preselected makes adoption of the system to useby a relatively large number of subscribers commercially feasible.

Referring now to Figs. 2 and 3, which should be joined left to right,respectively, a detailed showing is set forth of a portion of theswitching apparatus of the west terminal (Fig. 2) and a portion of theswitching apparatus of the east terminal (Fig. 3). In order to avoidconfusing duplication of similar apparatus, the detailed disclosure hasbeen confined, so far as clarity and completeness of description willpermit, to a showing of the transmitting switching apparatus of westsubscriber A (Fig. 2) and the receiving switching apparatus of eastsubscriber A1 (Fig. 3).

Subscribers set is `coupled to transmitting line 1| and receiving line12 by transformer 13, network 14 serving to provide a proper balance inthe well-known manner. Transmitting line 1| may be connected throughoperation of certain relays, which will-be described in detailsubsequently, to any one of the three west-east speech channels 24, 25or 26; receiving line 12 may be connected through operation of otherrelays to any one of the three east-west channels 4|, 42 or 43.

It will be observed that the relays of the transmitting switchingapparatus illustrated in Fig. 2 may, in general, be separated into threegroups, one group being associated with each west-east speech channel.For example, it will be apparent from the subsequent description thatrelays 15, [16,] 11, 8|, 82, 83, 84, 85 and 86 are associated withspeech channel 24; that relays ||l|, |62, |53, |04, |65, |06, |01 andare associated with speech channel 25; and that relays ||2, ||3, ||4,||5, |I6, ||1, |2| and |22 are associated with speech channel 26.

Speech energy emanating from subscribers set Il follows two parallelpaths, one path through delay network |24 and out over channel 24, 25 or26 depending upon which group of relays is operated, and the other paththrough amplifier-detector |25 to the various relay circuits.

In general, selection of the west-east speech channel is dependent uponthe position of brush |26 of rotary selector switch |21; operation ofthis selector switch when a speech fragment of vocal interval durationis applied to a channel is effective to preselect the next speechchannel in a manner that will be clear from subsequent description. Theswitch may be of the general type disclosed in O. F. Forsberg et al.Patent 1,472,465 and comprises two banks |3| and |32, two brushes |26and |33 cooperating therewith and stepping magnet |34. It will beunderstood that the two brushes are coupled mechanically so thatenergization of stepping magnet |34 causes rotation of both brushes.Selector switch |21 is common to all the [subscribers] subscriberstelephone circuits of the west terminal.

For purposes of further illustration of the invention the operation ofthe circuits of Figs. 2 and 3 will now be described in detail. In thecondition illustrated, relays 11, 8| and |31 (Fig. 2) are operated overenergizing circuits traced from battery |5|, brush |25 and first contactof selector bank |3I, line |52 and thence over two paths, one comprisingan armature and break contact of relay 82, the armatures and breakcontacts of similar relays in other subscribers circuits and thencethrough the winding of relay |31 to ground |53 and the other throughbreak contact of relay ||1, break contact of relay |65,

6 operating winding ofy relay 8| to ground |54 and operating winding ofrelay 11 to ground |55.

' `Relay |31 being operated, control tone of fre- ;quency fe produced bysource 55 is applied through band-pass iilter |56 to west-east controlsignal path 21 for a purpose which will be described in detailsubsequently.

Assuming now that subscriber A speaks, speech energy produced bysubscribers set is transmitted through transformer 13 to transmittingline 1|. As the energy is delayed somewhat in its passage to thewest-east speech channels by `delay network |24, let us see first whatactions result from passage of a portion of the energy throughamplifier-detector |25.

In view of the fact that relay 8| is in operated position, we haveobvious completed paths through the operating windings of relays 82, 83,84, 85, 86, [15 and 16] and 75 all of which relays are therefore nowoperated by the energy from amplifier-detector |25. An obvious path isalso completed from the battery 164 to the operating winding of relay 76by the operation lof relay 75 so that relay 76 is also in operatedposition.

Operation of relay 16 applies control tone of frequency f1 produced bysource 5| to west-east control signal path 21 through band-pass filter|61; the purpose of this control tone will be described subsequently.Operation of relay 66 connects transmitting line 1| to west-east speechchannel 24 so that the speech energy after traversing 'delay network |24is sent out over speech channel 24.

Operation of relay 82 releases relay 137 and establishes a lockingpath'over which relays 11 and 8| are held operated, this path beingtraced from battery |62, make contact of relay 11, make contact of relay82, line |63 and 'through the operating windings of relays 8| and 11 torespective grounds |54and |55.

Operation of relay 15 is4 effective to remove battery |64 from the makecontact of relay |65. Relays |55, |66 and |61 are associated with theswitching circuits of a second subscriber and correspond respectively torelays 11, |52 and ||3 of subscriber A. Relays |8|,v |82 and |83 arealso associated with the circuits of a second subscriber and correspondrespectively to relays 15, I3! and |I2. In the instance of relays |65,|66 and |51 howevena connection to battery is not obtained directlythrough a contact of the respective relay but only from the breakcontact or a relay in the circuits of subscribers set and, then, only ifsaid relay be in unoperated position. This arrangement is provided sothat evenv if two subscribers were to start talking at the saine instantthere would Abe no chance of the speech of both being placed on the samechannel. For example with the circuits in the condition just describedthere is no chance of a second subscriber breaking in on channel 24 withsubscriber A as operation of relay 15 has removed battery from the makecontact of relay |55` whereupon the holding circuit for relays in theother subscribers circuit corresponding to relays 11 and 8| cannot beestablished. In the instance of the switching circuits of the thirdsubscriber, the battery supply path also includes the break contacts ofrelays ISI, |82 and |83 of the second subscribers circuit; in theinstance of subsequent subscribers circuits the path includes the breakcontact of a corresponding relay of the preceding circuit. Thus, forexample, if thesecond subscriber (B)v andthe fourth subscriber (D) were`to start talking simultaneously. subscriber -A being idle, subscriber Bwould 'be .pu't on the next idle channel and the connection ofsubscriber D 'thereto at the `same time would 'be prevented due to thebreak in'the battery .supply path resulting from operation .of relay'|81 Operation of /relay 83 causes ra break in the path .connectingcertain of the relays ofthe group associated with channel 25 tothesecond contact of selec-tor'bank I3 I; operation `oi" relay 84 causes 'asimilar break in thegroup of relays associated with channel l26.

Operation of relay v85 vcompletes an Venergizing path for selectorstepping magnet |34 which may be traced from ground |84, make contactv.of relay 85, 'line |85, first Ycontact of selector bank |32, brush|33, operating =winding of stepping magnet |34 .toibattery |86. Brush'|33'is therefore stepped around to the second contact of bank 32 whereit comes to reset. 'Had this contact been lmarked busy by the connectionof rground thereto due to the .operation of relay |101 (oracorresponding -relay of a switching circuit of another subscriber) brush.|33 would, of course, continue its rotation .until it reached an idlecontact of bank |32.

As 'brushes |26 and |33 are coupled mechanically, rotation of ybrush |33to the second contact of lbank |32 will cause a corresponding rotationof brush |26 to Ithe second Contact of bank I3|. This rotation of brushr|26 to the second contact 'of ibank |3I is effective to preselect thetransmit- `ting vend of west-east channel 25 so that the next speechfragment of vocal interval duration pro- .duced by `any of the[subscribersfl subscribers sets vof the West terminal will be appliedthereto. This preselection of the transmitting end of Achannel 25 isaccomplished by setting Vup an en.- .ergizing path for those relaysassociated with other subscribers" vsets which correspond to relays I02and |03 of subscribersset II. Inv-the :instance .of Yrela-ys |02 yand|03 this energizing path is :prepared but not completed as relay 83 isInovv 1in .operated position; the path when completed, i. e., when relay33 -is deenergized, `may f be traced from battery I--.I, Vbrush |26,second Acontact of selector bank I 3I, line I9|, break contact .oflrelay -I I6, break -eo-ntact of relay l83, operating winding of :relay|03 to ground |92 ,and operating winding of relay |02 yto ground .1.03.With this condition, therefore, the next Yspeech fragment `Vof vocallinterval duration produced by vany of the other vwest subscribers setsis placed on .channel 2-5; .should lsubscriber A Epause long enough ,forthe relays associated with .channel .Z4 to .restore .to .normal positionbefore .any of the .other West Subscribersrstart talk.- ging, relays.|02 and |03 will operate `over the ,path described above and should hethen resume speaking the resulting speech fragment will b e placed onchannel .25.

Although battery .I5-I .Was removed from line '|52 as soon as brush I2,6 moved off .the irst contact of bank I 3|, relays .11 and 8| remain.operated due to .the locking vp ath established .by operation `of relay82.

We ,'have now seen how control .tones .of `fre- ;quency fa and .f1 havebeen Yapplied to west-east control signal path 21; how the speech energyproduced by subscribers set I I has been applied -to the transmittingend of West-east speech chan- *nel 24;k and how the transmitting end ofwest- 'east speech channel l2,5 has been preselected for applicationofthe next speech `fragment lof -vocal linterval duration. lLet usnowsee what has'been happening at the recei'vingendFig 3i)andparlticularly how subscribers set I6 has been connected to vthereceiving end of 4channel 24 and how the receiving `end of .channel 25has been preselected.

There is included in the switching apparatus illustrated in Fig. 3,channel selector 2II of subscriber A1, stepping selectol1 2|2 ofsubscriber A1 4and collecting selector 2 I 3 which is common to allofthe east subscribers sets. Brushes 2 I4 and 2 I5 of selector 2| I(which are coupled mechanically) `and brush 2|'6 of selector 2| 2 arecoupled mechanically and are rotated in step by selector stepping magnet2I1; brush v22| of selector 2I3 is rotated by selector stepping magnet222. The Acontacts of the bank of selector 21| 2 are connected inmultiple to contacts of the banks of the stepping selectors of the othereast subscribers.

It -will be recalled `from previous description of Fig. 2 that controltone of frequency f1 is being transmitted over control signal path 21 atthis time (when subscriber A is speaking). This control tone passesthrough band-pass ilter 24|, which is designed to pass only a narrowband of frequencies including f1, to amplifier-,detector 242. 4TheVenergy emanating from amplier-detector 242 causes the operation, overobvious paths, of relays 243, 244 and 245. Operation of relay 243 completes at its two make contacts la path' over which the speech currentsbeing received over speech channel 24 are applied to the receiving sideof the telephone circuit `of subscribers set I6. A portion of thisenergy passes through amplifier-detector 245 and causes operation ofrelay `241-which places a shunt on the transmitting side of thetelephone circuit of set vlli during the time vspeech ,is being receivedover the receiving side of -the circuit. The speech currents produced bysubscribers set |^I of the West lterminal are noW received bysubscribers set I6 of `the east terminal therefore.

Operation of relay 244 removes battery 25.I from o ne side of theoperating winding of stepping magnet 2I1 While operation of relay 2,45places ground 252 on the other side of .the winding of magnet 2 I1 andon the rst contact of the -bank of stepping selector 2I2 and on thecorresponding contacts of 'the banks of the Vstepping selectors of theother east subscribers.

Removal of battery Y25| from the operating path of stepping magnet 2I1prevents rotation .of selectors 2| 2 and 2II during the time speech andthe control tone are Abeing received .over channel 2,4. With respect tothe stepping selectors of the other east subscribers, however, (assumingthat VWest subscriber A is the only one talking) pres.- ence of ground252 0n the first .contacts of the stepping selector .banks completesenergizing paths through the respective stepping magnets (correspondingtomagnet 2 I1) to the respective batteries (corresponding to battery25|) and causes the stepping Ibrushes (corresponding to brush 2| 6) andthe two Ycoupled brushes of the channel selectors (corresponding tobrushes 2| 4 and 2I5-) to move to Vthe second position of the respectivebanks. Ground 252 is also connected -to 4the rst contact of the bank ofcollecting seflector 2I3 which causes the collecting selector also 'tomove to the second contact of its bank as battery 253 is connected toone side of the operating winding of stepping magnet 222. By the aboveaction, it will be obvious that the receiving end `of vspeech channel 25has'now been preselected.

l"1l-he above described condition prevails so long .as subscriber .A`alone is talking; let us assume g. novi? that subscriber A ceasestalking. The removal of control tone f1 from path 21 results in relays243, 244 and 245 dropping back to unoperated position.

Restoration of relay 245 to normal position removes ground 252 from thecontacts of the stepping selector banks and from one vside of thewinding of stepping magnet 2|1; restoration of relay 244 reconnectsbattery 25| to the other side of the winding of stepping magnet 2|1. Thebrushes of selectors 2|2 and 2 are now moved to the second position ofthe respective banks as an operating path for magnet 2|1 is completedtraced from battery 25|, break contact or relay '244, winding of magnet2|1, brush 2|6 and rst contact oi' selector 2|2, resistance 254 toground 255. It should be kept in mind that at this time relays 21| and212 are in unoperated position as the control tone fa, which is appliedto control path 2l whenever brush |26 (Fig. 2) is in its iirst position,is not now being transmitted. The brushes of selectors 2 l2 and 2li willstop in the second position therelore due to the fact that a highpotential is placed at the junction of resistance 213 and the secondcontacts of all the stepping selector banks by the connection thereto ofbattery 253 through resistance 214 and the winding of magnet 222 inparallel. The constants of the system are so arranged that the diierencebetween the battery voltage on stepping magnet 2 'l and the voltage atthe junction of resistance 213 and the second contact of the banks willbe less than the value required for stepping action. The above is truealso of collecting selector 2 I3 so that brush 22| remains in the secondposition; unless a dead ground (corresponding to ground 252) is placedon a contact of one of the stepping selectors, magnet 222 is in eectshunted by resistance 214 and the current through resistance 213 is notsufficient to cause the collecting selector to operate.

Recapitulating the detailed description of Figs. l and Z: thetransmitting switching apparatus of the west terminal operated to placethe speech energy produced by subscriber set on the transmitting end ofchannel 24 while the receiving switching apparatus oi the west terminaloperated to connect the receiving end of channel 24 through tosubscriber set l5 which is paired with subscriber set the transmittingswitching apparatus of the West terminal operated to preselect thetransmitting end of channel 25 for all west subscribers except the busysubscriber who was left on channel 24 while the receiving switchingapparatus of the east terminal operated to preselect the receiving endof channel 25 for all east subscribers except the busy subscriber whowas left on channel 24; and certain switching equipment of the busy westsubscriber was conditioned so that as soon as he stopped talkingpreselection of the transmitting end of channel 25 would be effectivefor his station also while, in view of the position assumed by thecollecting selector at the east terminal, the circuits of the busy eastsubscriber are so conditioned that, as soon as the speech energy beingreceived from the west terminal ceases, preselection of the receivingend of channel 25 becomes effective for this subscriber also. The aboveis on the assumption that only sets and I6 are busy.

Let us consider now the purpose of control tone fa which, it will berecalled, is applied to control signal path 21 whenever selector brush|26 is in engagement with the rst contact of selector bank |3l. Thepurpose of this control tone is, briefly, to insure that the brush oi'collecting selector 2|3 is in step with the operation of the system and,if not, to restore synchromsm at least once in each revolution. Let usassume that the circuitv is in the condition illustrated ln Figs. 2 and3 except for the fact that brush 22| of the collecting selector is onthe second contact of the bank instead oi' on the nrst contact asillustrated, i. e., in the assumed condition the collecting selector isout of step by one position. The control tone fa received over controlsignal path 2| is passed by band-pass filter 28|, which is designed topass a narrow band o1' frequencies including ia, to ampliner-detector282. The energy emanating from amplifier-,detector 282 causes theoperation, over obvious paths, of relays 21| and 212. Operation of relay21| connects battery 215 through resistance 215, which is of relativelysmall resistance, to the junction of resistance 254 and the firstcontact of collecting selector 2|3 thereby, in effect, designating thiscontact as the idle position. Operation oi relay 212 removes the shuntpath through resistance 2'l'4 so that the full potential o battery 253is now applied to magnet 222. If brush 22| is on the second contacttherefore, as we have assumed, it will be stepped around until itreaches the first contact thereby restoring synchronism. Upon reachingthe first contact it will stop due to the balancing potential appliedthereto through resistance 21 E. This same action will take place, ofcourse, with respect to the stepping selectors so that if any of themare idle and out of step they will also be collected and brought to thei-lrst position. Busy stepping selectors will not be aii'ected asoperation of the relay corresponding to relay 244 will in each case haveremoved the stepping battery.

This synchronizing action takes place once during each revolution ofselector |21, that is, whenever brush |25 is in engagement with the nrstcontact of bank I3|.

It will be understood, of course, and is so indicated schematically, inFigs. 2 and 3 that subscriber A is provided, in addition to thetransmitting switching apparatus which is illustrated, with receivingswitching apparatus (indicated by boxill) which is generally similar tothat illustrated in detail in Fig. 3 `and that subscriber Al isprovided, in addition to the receiving apparatus which is illustrated,with transmitting switching `apparatus (indicated by box Which isgenerallyA similar to that illustrated in detail in Fig. 2. It followsfrom this that transmission irom subscriber A1 to A (east to West) isaccomplished in a manner generally similar to west-east transmissionwhich has been described in detail above. During reception by subscriberA, a portion of the speech energy being received over line 12 is passedinto amplierdetectorl and the energy emanating therefrom causesoperation of relay Sir; operation of relay SI2 places a shunt ontransmitting line 'il during. reception over line 12.

While a specific embodiment of the invention has been selected fordetailed description it will be understood that the invention is notlimited in its application to the embodiment described. The embodimentdescribed should be taken as illustrative of the invention and not asrestrictive thereof.

What is claimed is:

1. In a speech transmission system, a plurality of transmitters, anequal number of receivers,

11. aV plurality of speech transmission channels, means for applying aspeech fragment of a` dura-1 tion equal to a vocal interval produced byone of said'transmitters to a. selected one of said speechv transmissionchannels, and means oper-v ated by energy resulting from production of aspeech fragment of vocal. interval duration by one of said transmittersto preselect both the sending end and the receiving end of an idlespeech channel for transmission of the next succeeding speech fragmentof such duration.

2. In a speech transmission system including a plurality oftransmitters, an equal number of receivers and a plurality of speechtransmission channels between said transmitters andI said receivers, themethod of maintaining secrecy during the transmission of speech fromsaid transmitters to said receivers which comprises impressing eachsuccessive speech fragment of a duration equal to a vocal intervalproduced by said transmitters on a dilerent one of saidj channels andpreselecting both the transmitting' end and the receiving end of an idlechannel for transmission of the next succeeding speech fragment of vocalinterval duration each time a speech fragment of such duration-1Sapplied t0 a channel.

3. In a speech transmission system, a plurality of transmitters, aplurality of receivers, a plurality of speech transmission channelsbetween said transmitters and said receivers, the number of saidchannels being less than that of said transmitters or said receivers,means operated by energy resulting from production of a speech fragmentof a duration equal to a vocal interval by one of said transmitters forconnecting the output thereof to a selected one of said channels, meansfor maintaining said connection during the duration of said speechfragment and means also operated by said energy for preselecting boththe transmitting end and the receiving end of an idle speechtransmission channel for transmission of the next succeeding speechfragment of such duration.

4. In a speech transmission system including a west terminal and an eastterminal, a plurality of transmitters at the West terminal, acorresponding number of receivers at the east terminal, a plurality ofspeech transmission channels between said terminals, means at the Westterminal operated by energy resulting from production of a speechfragment of a duration equal to a vocal interval by one of saidtransmitters for connecting the output thereof to the trans'- mittingend of a predetermined oney of said speech transmission channels, meansat said east terminal for simultaneously connecting the` input.

of a selected one of said receivers to the receiving end of the samechannel, means for maintaining said connections during the duration ofsaid speech fragment, means at said west' terminal also operated by saidenergy for preselecting the transmitting end of an' idle speechtransmission channel for application of the next succeeding speechfragment of such duration and' means at the east terminalA forsimultaneously preselectng the receiving end of that same channel thetransmitting end of which is preselectedY at the west terminal.

5. In a speech transmission system including a west terminal and an eastterminal, a plurality of transmitters at the west terminal, a corre-Ysponding number of receiversat the east lterminal, a plurality of speechtransmission channels between said terminals, means at said Westterminal forl applying. a speech fragment` ofvocal interval durationproduced by onev of. said., transmitters to a-selected one of saidspeechtransmission channels, means including a rotatable ele-f ment atsaid westterminal operated by energy resulting from. production ofv aspeech fragmenty ofY vocal.k interval duration by one of saidtransmitters to p reselect the transmitting end of an idleY speechtransmission channel for applicationY of thenextsucceedingspeechnfragmentlof such duration, means at said eastterminalincluding arotatable element for simultaneously preselecting the receiving end vof,the same idle speeehVV transmission channel, and means operating fonceduring each rotationof saidrotatable element of saidwest terminalpreselectingl means for bring,- in-g said tworotatable elements intopositional synchronism. l l v 6. In` a speech transmission systemvincluding awest terminal and an east terminal, a plurality oftransmitters at thewest terminal, a, correspending number of receiversat the eastterminal, a plurality of speech transmission channelsbetween. saidterminals, a control signal. path between said terminals,means operated by the energy resulting fromproduction of a speechfragment of vocal interval rduration by one of said transmittersI forconnecting the output of said transmitter tothe west terminal. end of aselectedA one of said speech transmission channelsmeans at said, westterminal for applyinga tone Signal of distinctive frequency path, meansat. the Veast terminal operated by said tone signal to connect the inputof a selected one of said receivers to the east terminal end ofproduction of said speech fragment to preselect` the west terminal endof another of saidspeech transmission channels for subsequenttransmission of speech energy produced by oneV ofA said transmittersand` means at said east terminal for simultaneously preselecting theeast terminal end of4 that speech transmission channel the westterminal. end of which was preselected by said previously mentionedmeans.

7.k In. a speech transmission system including apair of separatedterminals, a plurality of subscribers. telephone sets at each of saidterminals, a plurality of speech transmission channels be. tween. saidterminals, the number of said channels being less. t-han the number ofsaid sets, means for establishing varying interconnections between. saidsubscribers: sets and said.V channels whereby each successive speechfragment of a` duration equal to a-` vocal interval produced by any ofsaid setsYat-one of. said. terminals is placedl on adifferentspeechchannel than thaton whichl the preceding fragment of such duration. wasAplaced, said means operating eachrtime aspeech fragment. of vocalinterval duration is applied to oner of" said channels to preselect boththe sending endl and theA receiving end channel for transmission of thenext succeeding speech fragment of. such duration produced by anyof'said sets at saidv terminal.

8; In a speech transmission system including a west terminal and an eastterminal, a plurality to` said control signal of an idle speech mais '1Sduction of a speech fragment of vocal interval duration by a busy one otsaid transmitters for connecting the output thereof to the west terminalend of a predetermined one o! said speech transmission channels, meansat said east terminal for simultaneously connecting the input of aselected one of said receivers to the east terminal vend oi' the samespeech transmission channel,

means 'for maintaining said connections to both ends o! said channelduring the duration oi said speech fragment, means at said west terminalalso operated by said energy for preselecting the west terminal end ofan idle one of said speech channels for `transmission of the nextsucceeding speech fragmento! vocal interval duration pro- "duced by any'of the `other of said speech transmitters, and means'at said eastterminal for pre- -selecting the east terminal end -speech `channel forreception 'ny-any of the 'of said receivers,

of the same idle other said last-mentioned west terf nal means `beingeifectivealso to `condition circuits of said busy transmitter wherebyupon ter- I mination of the speech fragment produced thereby the westterminal end of said preselected -channel is made available thereto.said last-mentioned east terminal means being effective also tocondition circuits oisaid selected one oi said receivers whereby upontermination of the speech fragment produced by said busy transmitter theeast terminal end oi said preselected channel is lmade available to saidselected receiver.

'9. 'In a speech transmission system including if...

'a west terminal andan east terminal, a plurality ofspeech transmittersat the west terminal, a y I corresponding number of speech receivers atthe east terminal, a plurality of speech transmission channels betweensaid terminals, means operated by energy caused by production of aspeech fragment of vocal interval duration by a busy one oi. saidtransmitters for connecting the output thereof to the west terminal endof a predetermined one of said speech transmission channels and forsimultaneously connecting the inputfof a selected -;one of saidreceivers to the east terminal end of the same speech transmissionchannel, means also operated by said energy for placing the outputs ofall idle transmitters in connectible relationship to the west terminalend lof an idle one Tof said speech transmission channels and for:placing the inputs of all idle receivers in connectible relationship tothe east terminal end of Vfthe same last-mentioned channel'afnd meansoperated by .energy caused by production of a speech ragment of vocalinterval duration A.by one of said ast-mentioned transmitters forcompleting the onnection of the output thereof tothe west ter-l` minalend of said last-mentioned Achannel and or simultaneously completing theconnection or 'the input of a selected one of;said last-men-"25f'j5tioned receivers to the east terminal end of the samelast-mentioned channel, fsaisl last-menioned means also placing theoutputs of all transmitters still idle in connectible relationship to hewest terminal end of anotheridleone of said channels and the inputs ofall receivers still idle vr in connectible relationship to the eastterminal jend of the same last-mentionedchannel.

I DOREN MITCHELL.

No references cited.

